The present invention relates to a device for switching on and off an electrical switch assigned to control at least one functional unit of a knitting machine to select between two different operational conditions of the unit. The knitting machine includes a plurality of selectable needles, a generator for delivering timing signals for the knitting machine, a pattern controlling device for delivering binary control signals to selectively activate or inactivate the needles according to a predetermined pattern.
In circular or flat knitting machines, the switching devices of this kind activate or deactivate the knitting needles during a loop or mesh forming process, take up a thread and the like, and switch on and off the functional units, for example, the thread inserting members of a thread changing apparatus or a locking part. As a rule, the controlling signals are stored in a memory of the pattern controlling device, read out from the memory in synchronism with the cycles of the knitting machine and supplied to a selecting device for the needles or to a switching member for the functional unit and the like. Each pattern control signal causes immediately a predetermined action of the needle selecting device or of the switching member. All control signals form together a program assigned for a selected knitting pattern.
For other applications special devices are necessary which do not operate according to the predetermined program but always automatically switch on or off certain functional units when a predetermined sequence of the control signals occurs in the course of controlling an arbitrary pattern.
For example, in the production of patterns a reliable monitoring of a thread immediately at the point of knitting becomes difficult when the thread is being worked at some times and not being worked at other times and, in the latter case, is held by a pneumatically operating suction nozzle or a mechanical clamping device (DE-AS No. 11 48 347 and DE-AS No. 12 20 076). In the first mentioned case, a mechanical stopping device (DE-OS No. 25 56 387) for monitoring the presence of a thread causes the stoppage of the knitting machine only in the case when the thread is supplied in due order because its spring biased arm withdraws the thread from suction nozzle. By contrast, a stopping device reacting to the movement of the thread (DE-PS No. 15 60 582, DE-PS No. 29 07 653, French Patent No. 14 00 308, Swiss Patent No. 479 478 and Swiss Patent No. 596 078) stops the knitting machine independently of the condition of thread. Both in the case, when for the pattern forming purposes, the thread is temporarily held motionless by a mechanical or pneumatic clamping or holding device, and also in the case of malfunction, for example when the thread due to breakage is no longer moving, this stopping device stops the machine. The solution of such problems by prior art devices is not possible.
A different problem occurs at certain knitting machines for the production of plush knitwear (DE-OS No. 31 45 307). In such machines the temporarily unprocessed plush threads float behind those needles which process the plush thread. If the floating thread portions reach a certain length depending on the type of the knitting machine, then there is the danger that the plush loops present at the beginning of the floating portions are shortened when the formation of the plush loops is resumed at the beginning of the floating portion. To avoid this problem it has been proposed to equip the knitting machine with the so-called thread wheels which clamp the floating thread portions between its spurs and the plush loops, thus preventing the subsequent shortening of the previously formed plush loops. For operational reasons it is desirable to inactivate the thread wheels by switching members such as electromagnets and the like, when the floating thread parts reach a certain minimum length, so as to be able to cut the threads in a conventional manner in order to avoid excessively long floating parts and to clamp the thread by a pneumatic or mechanical means (DE-OS No. 38 12 124). Device for solving this problem are not yet known.